In comparison to turbojet transportation, propeller driven aircraft, such as piston-engine and turboprop aircraft, have historically been considered less comfortable from a noise and vibration standpoint. In a propeller driven aircraft, the propellers tend to contribute noise and vibration at a frequency referred to as the “blade passage frequency” and at harmonics of that frequency. As referred to herein, the blade passage frequency or “BPF” is the product of the propeller shaft rotational speed times the number of blades on a propeller. For example, an aircraft having a 3-blade propeller on a shaft turning at 2000 revolutions per minute has a BPF of 3×2000 or 6000/minute, i.e., 100 Hz. Most commonly, all of the propellers on a multi-propeller aircraft have the same number of blades and operate at the same rotational speed, so that there is only a single BPF for the entire aircraft. Typical aircraft have at least one preferred cruise setting, at which the propellers will operate at a particular rotational speed. Thus, there will typically be a single BPF for the entire aircraft corresponding to that preferred cruise setting.
FIG. 3 is a graph showing representative interior noise levels in one type of propeller aircraft at high speed cruise. As shown in the graph, the sound pressure level spikes at the BPF and successive harmonics thereof. This aircraft has a BPF of approximately 100 Hz. The interior sound pressure level spikes at a frequency equal to the BPF, also referred to herein as the “first harmonic of the BPF.” The sound pressure level also spikes at the second harmonic of the BPF, or approximately 200 Hz, and at the third harmonic of the BPF, or approximately 300 Hz, and so on. As the graph shows, the highest two spikes in sound pressure level are at the first and second harmonics of the BPF.
Over the past several decades, considerable effort has been expended developing systems to attenuate some of the undesirable noise and vibration in propeller driven aircraft. Major considerations in the development of noise and vibration control systems include keeping added weight and cost to a minimum, while maximizing attenuation of unwanted noise and vibration.
Such noise and vibration control systems are generally classified as either active or passive. Active systems comprise using secondary control sources to add additional energy to a vibrating system to cancel out the primary excitation. For example, active noise control comprises using acoustic sources, such as loudspeakers, to cancel targeted sound within the aircraft coming from the propellers. Active structural acoustic control, on the other hand, comprises using vibration inputs, such as shakers or piezoelectric materials, to modify the sound field in the aircraft. Another technique includes “synchrophasing,” which includes adjusting the relative rotational phase of the propellers in a multiple-propeller aircraft to reduce interior noise.
Passive systems, on the other hand, do not require a power source to provide energy to the system. Passive techniques include providing damping material, such as thermal/acoustical insulation blankets, along the interior of the aircraft fuselage to muffle sound transmission into the interior of the aircraft. Other passive systems include providing vibration absorbers to attenuate vibration of the fuselage structure. For example, one such prior art system (as shown in FIGS. 1 and 2) includes mounting tuned vibration absorbers (“TVAs”) to the frames of the fuselage to attenuate vibration of the fuselage structure.
FIGS. 1 and 2 illustrate portions of the interior of the fuselage of the King Air 350 model turboprop aircraft manufactured by Hawker Beechcraft Corporation. The fuselage comprises a series of frames 10. Each frame 10 is generally in the form of a ring that extend around the fuselage in the circumferential direction. The frames 10 are spaced apart along the longitudinal extent of the aircraft and are interconnected by a series of stringers 12, which run along the longitudinal direction of the aircraft, transverse to the frames 10. The frames 10 and stringers 12 are connected to the skin 14 of the aircraft, which forms the exterior surface of the aircraft fuselage and which encloses the interior volume of the aircraft.
Vibration attenuation systems 16 are attached to the frames 10 for lessening vibration of the fuselage. Each attenuation system 16 comprises first and second TVAs 18, 20 connected to a mounting bracket 22. Each mounting bracket 22 is secured to a fuselage frame 10 such that the TVAs 18, 20 are positioned adjacent to the frame 10. The attenuation systems 16 are generally arranged in pairs at each frame 10, as shown in FIG. 2, with attenuation systems 16 being attached to both the forward and aft sides of the frames 10. The TVAs 18, 20 of each attenuation system 16 are arranged to attenuate vibration of the frame 10 in the direction normal to the aircraft fuselage. Each TVA 18, 20 is a mass and spring system. The mass is configured to move towards and away from the central longitudinal axis of the fuselage to attenuate vibration in that direction. Specifically, each of the TVAs 18, 20 includes a spring in the form of an elongated plate 24 connected to the mounting bracket 22 at approximately the center of the plate 24 and having masses 26 connected at each end of the plate 24. The plate 24 is flexible and permits the masses 26 to move towards and away from the central longitudinal axis of the fuselage in response to vibration of the frame 10 along that direction, which vibration is transmitted to the TVAs 18, 20 through the mounting bracket 22. Each of the two TVAs 18, 20 is designed to be tuned to a different frequency. In particular, the first TVA 18 is tuned to 100 Hz (i.e., the first harmonic of the BPF) and the second TVA 20 is tuned to 200 Hz (i.e., the second harmonic of the BPF).
Despite the above progress in the art, further improvement is still desirable.